Separating a stream containing a multi-phase mixture and comprising lighter and heavier density liquids and particles entrained therein

ABSTRACT

A separation method and apparatus are disclosed having particular application for effecting removal of sand from a production wellstream, enhancing the quality of recovered oil, gas and water output streams ( 12, 13, 14 ) and reducing erosion caused by entrained sand. The wellstream is passed initially through a cyclone separator ( 4′ ) which separates substantially all the water and sand as underflow and substantially all the oil and gas as overflow. A gravity separator ( 11 ) separates the overflow from the cyclone separator ( 4′ ) into oil, gas and water, and a further cyclone separator ( 17 ) separates the underflow from the first cyclone separator ( 4 ′) into water and sand.

[0001] This invention relates to a method of separating a hydrocarbonproduction fluid stream from a production well, which stream contains amulti-phase mixture comprising oil and water and particles entrainedtherein, the mixture being rich in oil. More particularly, though notexclusively, the invention relates to a method of separating ahydrocarbon production stream (particularly when produced by waterinjection into a production well) into gas, oil and water streams, andsand or other entrained particles. It also relates to an installationfor separating hydrocarbon production fluid comprising a multi-phasemixture of oil and water and particles entrained therein into individualconstituents of said production fluid, said hydrocarbon fluid being richin oil.

[0002] A production stream generally contains crude oil and hydrocarbongas as principal valuable products, and also water and sand/silt (orother entrained particles, such as rock fragments) water is usuallypresent naturally. However, where water is injected into the well toenhance recovery of oil and/or gas, the proportion of water present inthe production stream will be higher than it the only water present isthat occurring naturally in the subsea oil reservoir. Normally, thewater and oil will be present in the production stream as a oil-in-wateremulsion, i.e. the water forms a continuous phase and the oil adispersed phase, or a water-in-oil emulsion, i.e. the oil forms acontinuous phase and the water is a dispersed phase. Furthermore, gas,when present, will exist as a separate dispersed phase, i.e. as bubbles.Some of the gas will normally be dissolved in the liquid phases, theamount of dissolved gas varying according to the pressure of the streamat any point in question. The sand which is present typically existsnaturally in the subsea reservoir, along with silt, and is normallysupplemented by further sand produced as a result of the drillingoperation carried out in the subsea earth formation, the action of waterinjection on unconsolidated formations, and gas and liquid pressuredifferentials acting across the production formation. The sand and siltor other particles are entrained in the phase mixture constituted by theoil, gas and water.

[0003] It is known to separate the components of a production wellstreamusing a gravity separator, in order to recover oil and gas as valuableproducts. Such a separator is essentially a large storage vessel or druminto which the wellstream is introduced and allowed to settle. Gravitycauses the denser, generally dispersed, water phase to coalesce andsettle to form a layer at the bottom of the separator, and the lessdense, continuous, oil phase to form a liquid layer on top of the waterlayer. Hydrocarbon gas, present as a dispersed phase (bubbles) and insolution, separates from the water and oil and fills the atmosphere inthe space above the liquid phase layer. Water, oil and gas can be tappedoff periodically or continuously from the water and oil layers, and gasatmosphere, respectively.

[0004] Such a separator operates in a reasonably effective way toseparate a hydrocarbon oil stream into its constituent parts. However,in certain situations, oil and water can form emulsions which aredifficult to break in a conventional separator. Furthermore, the sandand silt present in the wellstream accumulate on the bottom of thegravity separator and have to be removed periodically. In addition, thesand and silt present in the wellstream cause erosion-of the pipes alongwhich is conveyed to the gravity separator, of the separator itself andof other components, such as chokes and control valves. Such pipesusually include a riser, connecting the wellhead manifold on the sea bedto the gravity separator which is usually onshore, or on a floatingsupport vessel, rig, or production platform. Conventional thinking is touse a desander which removes the sand and silt in the wellstream beforeit is introduced into the gravity separator. One conventional form ofdesanding plant of this kind is shown in FIGS. 1 and 2 of theaccompanying drawings.

[0005]FIG. 1 is a schematic view of one form of conventional desandingplant for removing sand from a production stream and separating theproduction stream into gas, crude oil, water and sand. In this Figure,an inlet manifold 1 receives a production fluid stream from one or moreproduction wells through respective lines, one of which is indicated byreference numeral 2. The manifold 1 is situated on the sea bed, thoughit could instead be located on a production preparation platform or at alocation intermediate that platform and the sea bed. It is connected bya riser 3 to a demanding plant 100 to be described, which is eitheronshore or carried by a support structures such as a floating vessel orrig, or a structure secured to the sea bed.

[0006] The mixture flowing in riser 3 is introduced into a cycloneseparator 4. Cyclone separators as such are well-known in the art andrely on generated centrifugal and shear forces to achieve separationinto two streams of different densities.

[0007] Briefly and as shown in FIG. 2, the cyclone separator comprises achamber 40 having a vertical axis with an upper cylindrical portion 40 aand a lower, inverted frustro-conical portion 40 b. The mixture isintroduced through a tangential inlet nozzle 41 to the cycloneseparator, which causes heavier particles (wet sand/silt) to be flung,under centrifugal force, against the outer wall of the chamber and flowdownwardly along, as underflow, and around the wall to a lower axialoutlet 42, while the lighter, remaining, proportion of the mixture isdrawn off by an axial pipe, known as a vortex finder, from a pointwithin the body of the cyclone separator 4 as overflow and conveyedoverhead through upper axial outlet 43. Suitably, the, cyclone separatormay be of the form disclosed in U.S. Pat. No. 4,737,271, but other formsof cyclone separator, such as are well known in the art, may be usedinstead.

[0008] Of the constituents of the production stream, the sand and siltparticles have the highest density, followed by the water, then oil andlastly the gas, which has the lowest density. The cyclone separator 4 isset up such that, essentially, it removes the sand from the stream,leaving oil, gas and water which passes out through the upper axialoutlet 43 of the separator. In practice, though, the sand will contain aproportion of water, so that it leaves cyclone separator 4 via loweraxial outlet 42 as wet sand in the form of a slurry. The wet sand ispassed directly through line 5 into a sand accumulator 8, which takesthe form simply of a large storage vessel. In this vessel, the sandbuilds up as a layer on the bottom of the storage vessel, with watersettling out above. As the level of the sand rises, the sand displacesthe overhead water upwardly so that, in effect, there is a discharge ofwet sand from the underflow outlet 42 into the accumulator and acountercurrent migration of water in line 5, from sand accumulator 8back up to the cyclone separator 4, where this water, as it is madeavailable, is separated from the sand along with oil and gas and passedout through overflow axial outlet 43. Periodically, sand may be removedfrom sand accumulator 8, as indicated by reference numeral 9, to makespace available for the accumulation of further sand in the accumulator8. If desired, the sand may be cleaned and rinsed, ready for dumping.

[0009] The oil, water and gas mixture leaving cyclone separator 4through its upper axial outlet 43 passes along line 10 to a gravityseparator 11, which separates the mixture into its three constituents,oil, gas and water. The gas and oil are the valuable constituents whichit is desired to recover. The produced water is a product having noparticular value since its purity is not sufficiently high for manypurposes. For example, it will contain traces of oil, which means thatfar environmental reasons the water cannot be discharged directly intothe sea or into a local public sewer. Typically, the produced water willbe used as injection water to be re-injected into the well to aidrecovery of further oil and gas.

[0010] The gravity separator 11 is of known construction and comprisessimply a large tank or drum into which the oil-water-gas mixture isintroduced. In the container, the water droplets constituting the waterphase coalesce to form a body or layer of water which settles undergravity to the bottom of the container, underlying the larger body ofoil that forms on top of the water. The gas bubbles coalesce to form,along with gas coming out of solution, a gas atmosphere filling theremaining space inside the container above the body of oil. A gas line13 is connected to this atmosphere, and oil and water lines 12, 14 arerespectively connected to openings in the wall of the container suchthat they communicate with the oil and water layers, respectively. Thepressure in gravity separator 11 is controlled by the setting of valve51 in gas line 13. The gravity separator 11 is a crude but reasonablyeffective device for effecting the necessary separation between the gas,oil and water.

[0011] The cyclone separator is used, for the reason that a cycloneseparator is a simple, reliable and relatively inexpensive piece ofequipment that is highly effective in separating lower and higherdensity materials in the input feed. Therefore, the wet sand containsrelatively little oil or gas and the mixture in line 10 containsrelatively little sand and a relatively high percentage of the waterfrom the production stream in riser 3. Furthermore, since conventionalthinking is that the purpose of the cyclone separator is essentially toremove the sand so that the downstream components of the plant areprotected from the corrosive action of sand, the plant is designed suchthat the cyclone separator 4 functions primarily to remove sand only.Although the sand removed from the sand accumulator 8 will contain acertain amount of water or moisture, the water content of the sanddischarged from the sand accumulator is made to be as low as possible,commensurate with the design of the demanding plant.

[0012] For any given cyclone separator there is a separation or cutpoint, which determines which constituents of the inlet stream aredirected into the upper axial outlet for lower density constituents andwhich are directed into the lower axial outlet for higher densitymaterials. The separation point corresponds to a density which has anequal probability of exiting from either axial outlet. Higher densityconstituents will normally mainly leave the lower axial outlet asunderflow though the overflow will also then include a small amount ofthe higher density constituents. For increasing densities of higherdensity constituents, the proportion in the underflow will increasewhile that in the overflow will decrease. Correspondingly, lower densityconstituents will generally mainly leave the upper axial outlet asoverflow, though the underflow will also then include a small amount oflower density constituents for reducing densities of lower densityconstituents, the proportion in the overflow will increase while that inthe underflow will decrease. In this way a separation is effected asbetween constituents of different densities by appropriate selection ofthe separation point.

[0013] The most significant parameters determining the separation pointare the internal diameter of, the vortex-finder, around which thetangential inlet stream enters the cyclone separator, and the inletpressure. The vortex-finder diameter is a design parameter of thecyclone separator and the inlet pressure is adjusted by a control valve15 which is located either in riser 3 or in line 10. It also followsthat where a cyclone separator is to be used to effect a separationbetween two phases of a phase mixture of significantly differentdensities, one of the two output streams produced by the cycloneseparator can be caused to contain essentially no amount of one of thetwo phases by setting the cut point to a density that is between thoseof the two phases tinder consideration but nearer to that of thatone-phase.

[0014] Reference is now made to FIG. 1a. This Figure indicates therelative densities ρ of sand, water, oil and gas. The actual values ofdensity for the sand, oil and gas in the production stream will varyfrom well to well, possibly even with time for the same well, but therelative densities shown are typical. Also as shown, the cut point [4]of cyclone separator 4 corresponds to a density between those for sandand water. On the one hand, the cut point [4] is chosen to correspond toa density that is as small as possible relative to the density of sandno as to minimise the carry over of sand into the overflow stream. Onthe other hand, the nearer it comes to the density of oil and gas, thegreater the quantity of trace oil and gas in the underflow from thecyclone separator. The actual choice of the cut point is therefore atrade-off between these two conflicting operating requirements.

[0015] Whilst the described conventional desanding plant is reasonablyeffective in removing sand and water from the valuable oil and gas, asindicated above, a small proportion of the sand remains in the oil andgas stream leaving as overflow. This entrained sand passes to thegravity separator 11. The presence of this sand in the gravity separatorthen occupies part of the separator capacity. Furthermore, its presencereduces the quality of the final separation products. Still further, thepresence of unwanted sand in line 10 and gravity separator 11, albeit intrace amounts, causes erosion of the wall of line 10 and of theinternals of gravity separator 11. A need therefore exists for adesanding plant which is more effective in the removal of sand, in whichthe final output gas, oil and water lines are of improved purity and inwhich the erosion of components of the plant is reduced or substantiallyeliminated.

[0016] Reference is now made to a selection of prior art references.

[0017] U.S. Pat. No. 5,350,525 (Shaw et al) discloses a system forseparating a multi-phase liquid mixture, such as production hydrocarbonsfrom a well, into a plurality of streams. Following an initialseparation in a 3 phase gravity separator to produce a gas stream, anoil stream and a water stream (including small residual amounts of oiland sand), the oil-lean water stream is subjected, to a treatmentprocess in which it initially has heavier sand particles removed, isthen passed through a liquid-liquid hydrocyclone to remove the oil andlastly is passed through a solid-liquid hydrocyclone to form aparticle-rich slurry as underflow and solids-free water as overflow.

[0018] In U.S. Pat. No. 5,021,165 (Kalnins), an oil-in-water feed to ahydrocyclone is separated into oil and water. A further hydrocyclone canbe used to remove solids from the water. Residual oil in the water isseparated using a flotation cell.

[0019] U.S. Pat. No. 4,399,041 (Rappe) discloses the use of two stagedhydrocyclones to provide staged separation of solids.

[0020] With reference to U.S. Pat. No. 3,764,000, production fluid isinitially subjected to separation into gas, oil and water (containingoily sand), the latter being treated in staged hydrocyclones to separateoily water and de-oiled sand, leaving clean water.

[0021] Reference la also made to WO-A-97/11254 (Baker Hughs Ltd) inwhich staged hydrocyclones are used to separate firstly the oil from thebulk of the water and sand of a production fluid and secondly the bulkof the sand from the water, leaving a solid depleted water stream.

[0022] According to the invention from one aspect, there is provided amethod of separating a hydrocarbon production fluid stream from aproduction well, which stream contains a multi-phase mixture comprisingoil and water and particles entrained therein, into individualconstituents of said production fluid, said mixture being rich in oil,said method comprising the steps of passing the stream into a firstcyclone separator having a cut point corresponding to a density betweenthe densities of the oil and heavier water to produce an underflow thatis rich in heavier water and particles and an overflow that is rich inoil, passing the overflow from the first cyclone separator to a firstseparation means to effect a separation into oil and water, and passingthe underflow to a second separation means to effect a separation of theunderflow into a flow that is rich in water on the one hand andparticles on the other hand.

[0023] Since the separation (cut) point of the cyclone separator is setto correspond to a density between the densities of the oil and waternot only the particles (normally sand) but also substantially all the(less dense) water is included in the underflow and the effectiveness ofseparation of the particles from the oil is enhanced. Furthermore, theinternal components of the demanding plant receiving the overflow fromthe cyclone separator, in particular those of the first separationmeans, are less prone to erosion. In addition, the method provides asimple and effective way of breaking the multi-phase hydrocarbonmixture, so as to coalesce the dispersed phase.

[0024] Where the stream further includes a hydrocarbon gas phase, thestream, including the gas phase, may be passed initially into a secondcyclone separator to effect a separation into an overflow that is richin gas on the one hand and an underflow that in rich in multi-phasemixture and entrained particles on the other hand, the underflow rich inmulti-phase mixture and particles then being introduced as feed into thefirst cyclone separator.

[0025] Suitably, a separation of the underflow from the first cycloneseparator is effected by a third cyclone separator of the secondseparation means into an overflow containing a major proportion of waterand substantially no particles and an underflow containing a minorproportion of water and particles, and a separation of the underflowfrom the third cyclone separator is effected by a particle accumulatorof the second separation means into particles on the one hand and wateron the other hand.

[0026] Conveniently, the first separation means effects separation intooil, water, and, where applicable, gas by gravity separation.

[0027] In one preferred way of putting the invention into effect, thefirst cyclone separator has its cut point corresponding to a densitycloser to that of the water so that the overflow from the first cycloneseparator contains a minor proportion of water and its underflowcontains substantially no oil, and the overflow from the third cycloneseparator is combined with the water produced by the first separationmeans.

[0028] In an alternative way of putting the invention into effect, thefirst cyclone separator has its cut point corresponding to a densitycloser to that of the oil so that the underflow from the first cycloneseparator contains a minor proportion of oil and its overflow containssubstantially no water, and the overflow from the third cycloneseparator is combined with overflow from the first cyclone separator.This latter method has the advantage that since the amount by which thecut point of the first cyclone separator is below the density of sand isgreater than in the case of the preceding way of performing theinvention, any trace amount of sand in the overflow from the firstcyclone separator is further reduced.

[0029] In a further way of putting the invention into effect, the firstcyclone separator may have its cut point corresponding to a densitycloser to that of the oil so that the underflow from the first cycloneseparator contains a minor proportion of oil and its overflow containssubstantially no water. Then, a separation of the underflow from thefirst cyclone separator is effected by the second separation means intoa flow of water containing said minor proportion of oil on the one handand particles on the other hand, and a separation of the flow from thesecond separation means is effected by a fourth cyclone separatoressentially into water on the one hand and oil on the other hand.

[0030] It is preferred that measurements are made indicative of thequality of one output stream roam a said cyclone separator and themeasurements are used to control a parameter (for example flowrate) ofthe overflow from that cyclone separator to increase the quality of oneoutput stream or the other from that separator. By optimising quality inthis way, the separation method can produce separation products, atleast some of which are of high quality. Furthermore, where for examplea flow control valve is used to adjust the flow rate, this valve beinglocated in the overflow line, that flow control valve is not exposed tosaid erosion. In general, it is also possible to effect control of acyclone separator at an upstream location according to measuredparameters of the constituents of the streams at downstreamlocations(s), such parameters being flowrate, pressure or volumetricsplit ratios between the constituents.

[0031] In a preferred arrangement where the controlled parameter is theflow rate of the overflow from at least one cyclone separator, ouch flowrate is additionally controlled in dependence on the pressure existingin at least one flowline containing one or more of the constituents ofthe stream.

[0032] The invention is also concerned with an installation forseparating hydrocarbon production fluid comprising a multi-phase mixtureof oil and water and particles entrained therein into individualconstituents of said production fluid, said hydrocarbon fluid being richin oil.

[0033] According then to the invention from a second aspect, there isprovided an installation for separating hydrocarbon production fluidcomprising a multi-phase mixture of oil and water and particlesentrained therein into individual constituents of said production fluid,said hydrocarbon fluid being rich in oil, comprising a production wellfor producing said hydrocarbon production fluid stream, a first cycloneseparator connected to receive said production fluid stream as feed fromsaid production well, said cyclone separator having a cut pointcorresponding to a density between the densities of the oil and water toproduce an underflow that is rich in water and particles and an oil-richoverflow, a first separation means arranged to receive the overflow fromthe first cyclone separator an teed and to separate the same intolighter density liquid and heavier density liquid and a secondseparation means for effecting a separation of the underflow into awater-rich flow on the one hand and particles on the other hand.

[0034] Preferably, the installation is located on a seabed adjacent aproduction fluid wellstream manifold, so as to minimise exposure to theerosive effect of sand/silt or other particles in the production fluidfrom the source.

[0035] For a better understanding of the invention and to shoe how thesame may he carried into effect, reference will now be made, by way ofexample, to the accompanying drawings, in which:

[0036]FIG. 1 is a schematic view of a conventional desanding plant forremoving sand from a production stream and separating the productionstream into gas, crude oil and water;

[0037]FIG. 1a is a diagram useful for understanding operation of theplant according to FIG. 1;

[0038]FIG. 2 is a perspective view of a cyclone separator included inthe desanding plant;

[0039]FIG. 3 is a corresponding schematic view of a first embodiment ofthe invention;

[0040]FIG. 3a is a diagram useful for understanding operation of thefirst embodiment;

[0041]FIG. 4 is a modification to the first embodiment and

[0042]FIG. 4a is its corresponding operation diagram;

[0043] FIGS. 5 to 7 are corresponding views of second to fourthembodiments, respectively, of the invention and FIGS. 5a to 7 a theircorresponding operation diagrams; and

[0044]FIG. 8 is a schematic representation of a plant that is similar tothat of FIG. 7, additionally showing the manner of control of each ofthe cyclone separators included in the plant.

[0045] In the description which follows, like reference numerals denotethe same or corresponding components as between different embodimentsand also in relation to the conventional desanding plant according toFIGS. 1 and 2. Furthermore, a description is given only of thosecomponents and those operational features that differ from thecorresponding ones in FIGS. 1 and 2.

[0046] A first embodiment of the invention is shown in FIG. 3. In thisembodiment the production stream passes from manifold 1 to a firstoil/water cyclone separator 4′ via line 3. As in the conventional systemaccording to FIG. 1, the line 3 may be in the form of a riser and thedemanding plant to be described can be located onshore, on a floatingsupport or on a platform that stands on the sea bed. Alternatively, thedesanding plant may be located on the sea bed, in which case the line 3will be simply a short pipeline.

[0047] The cyclone separator 4′ differs from the cyclone separator 4 inthe conventional desanding plant according to FIG. 1 in that it is setup with a different separation point for the cyclone separator 4′, asindicated in FIG. 3a (see [4′]). Specifically, the cyclone separator 4′is set up such that its cut point corresponds with a density nearer thatfor water than that for oil. As a result, not only the sand present inthe stream supplied by line 3 but also substantially all the water isseparated as underflow in the cyclone separator but a minor proportionof water will be included with the oil and gas which pass upwardly asoverflow (overhead). Although it is desirable to minimise the watercontent of the oil-gas mixture flowing in line 10, nevertheless, it isimportant that the presence of some water in line 10 be accepted, sincethen it will be known that there can be even less oil, or to allpractical intents substantially no oil, in the underflow stream becausethe cyclone separator effects separation into two streams according todensity. In view of the different separation point chosen for thecyclone separator 4′, in particular one which separates not only thesand but also (less dense) water or, expressed another way, since thedifference between the density corresponding to the cut point of cycloneseparator 4′ and the density of sand is larger than in the case of thedesanding plant of FIG. 1, a higher proportion of sand is removed thanin the case of the conventional desander according to FIG. 1.Furthermore, particularly when siting the plant on the sea bed and asclose as possible to the manifold 1 (i.e. the line 3 will be of veryshort length), substantially all the sand is removed at the point ofentry into the desanding plant 100 and as close us possible to thewellstream manifold, so that as short a length of line 3 as possible isexposed to sand erosion, and the line 10 and all downstream equipment,especially the gravity separator 11, are largely protected from erosion.

[0048] The sand-water slurry or stream exiting the cyclone sand/waterseparation means, comprising cyclone separator 17 and sand accumulator8. The second cyclone separator 17 separates the sand and water, thesand with a small proportion of water exiting as underflow (wet sand)along line 5 to sand accumulator 8 with counter-current migration ofwater displaced from the sand accumulator back up line 5 to the cycloneseparator 17 through its lower outlet, and the water separated incyclone separator 17 exiting through its upper outlet of the cyclone asoverflow and passing along water line 6.

[0049] The density corresponding to the cut point [17] for the cycloneseparator 17 is indicated in FIG. 3a, the cut point density value chosenis a trade-off between being as far below the density of the sand/siltas possible, so that as near to 100% of the sand present in the feed tothe cyclone separator 17 is separated and directed into the underflow,and being sufficiently above the density of water such that an adequateproportion of the water from the feed is directed into the underflow toform a sand-water slurry of sufficient mobility that it can pass,without forming a blockage, through line 5 to sand accumulator 8.Normally, the cut point for the cyclone separator 17 will be somewhatcloser to the density of water than that of the sand/silt, as shown inFIG. 1a.

[0050] In the gravity separator 11, a separation is effected just as inthe known desanding plant according to FIG. 1 as between the gas, oiland water delivered through line 10, though in this case water ispresent only in trace amounts. The separated water is drawn off alongline 18 and combined with the water in line 29, to form water flow inwater line 14.

[0051] Lines 10 and 6 include respective control valves 19 and 20 whichare used for adjusting the flow rates in these lines, for controllingthe separation that is effected in the cyclone separators 4′, 17. Themanner in which such control is effected is described hereinbelow, withreference to FIG. 8.

[0052]FIG. 4 shows a modified desanding plant. In this embodiment, asshown in FIG. 4a, the cut point [4′] of cyclone separator 4′ isdifferent from that in the FIG. 3 embodiment. Specifically, itcorresponds with a density that is nearer to that of the oil than thatof water, so that the underflow from cyclone separator 4′ contains aminor proportion of oil but its overflow contains substantially nowater. In the sand/water separator 17, whose cut point is the same as orsimilar to that used in the FIG. 3 embodiment, the small amount of oilpresent in the feed is separated and discharged, along with a majorproportion of the water from the feed, as overhead. In view of thenon-negligible oil content, line 6 from the upper outlet of cycloneseparator 17 conveys the overflow (overhead) of that separator to line10 (rather than directly to water line 14), so that the overflow fromcyclone separator 17 mixes with the overflow from cyclone separator 4′,the mixture then being introduced into gravity separator 11 forseparation into oil, gas and water.

[0053] Reference is now made to the embodiment of FIG. 5, which issimilar to that of FIG. 4 but uses an additional water/oil cycloneseparator 22 rather than increase the loading on gravity separator 11.The cut point [22] for the further cyclone separator is the same as orsimilar to that of the gas/oil cyclone separator, 4′, as shown in FIG.5a.

[0054] More specifically, since the separation point of the firstcyclone separator 4′ is set as in the FIG. 4 embodiment such that notonly the water and sand is separated from the incoming productionstream, but also a minor proportion of the oil, the underflow leavingthe lower outlet of the cyclone separator 4′ consists of oily water andsand and the overflow leaving the upper outlet consists essentially onlyof oil and gas (negligible water is present). The presence of oil in theunderflow is taken as confirmation that no water (or sand) is present inthe overflow. The underflow of oily water/sand slurry passes along line16 to cyclone separator 17 which separates the slurry into wet sand asunderflow which passes along line 5 to sand accumulator 8, and oilywater as overflow through the upper outlet of the cyclone separator.This mixture then passes along line 21 to the inlet to third cycloneseparator 22 which effects a separation of the oily water into oil andwater, the separated oil passing as overflow through the upper outlet ofthe cyclone separator along line 23 to be combined with the separatedoil discharged along line 29 from gravity separator 11, to form a flowof oil in oil line 12. The separated water phase in cyclone separator 22is discharged as underflow through the lower outlet of the cycloneseparator and passes along line 14. Line 18 from gravity separator 11 isprovided as a contingency in case any water should find its way into thegravity separator, in which case line 18 conveys that water to the waterline 14.

[0055] With this embodiment, improved purity of the separatedconstituents of the production stream can be achieved with furtherreduced exposure to corrosive action of entrained sand.

[0056]FIG. 6 shows a third embodiment in which, in effect, the firstcyclone separator 4′ of the first embodiment according to FIG. 3 isprovided with a supplementary gas/oil cyclone separator (4″), whichseparates most of the gas from the teed. The cut point of the cycloneseparator 4″ is shown in FIG. 6a at [4″].

[0057] More specifically, the first cyclone separator 4″ separates gaswhich is discharged through the upper, overflow outlet and flows alongline 26. The underflow of the cyclone separator 4″ exits through itslower axial outlet as an oil/water/sand mixture (including a minoramount of gas), which passes through line 27 to the tangential inlet ofcyclone separator 4′. In this cyclone separator, oil and water and theminor amount of gas are discharged as overflow from the upper outlet ofthe cyclone separator and pass through line 28 to the gravity separator11. Oil that collects in the separator 11 is discharged along oil line12. Collected water is discharged along line 18. The gaseous atmosphereabove the oil layer in gravity separator 11 is connected by line 7 toline 26 and the combined gas flows pass along line 13. The underflowfrom cyclone separator 4′, which is a mixture of sand and water, ispassed through line 16 to cyclone separator 17 which separates the wateras overflow and wet sand as underflow, which is discharged from thelower outlet of the cyclone separator into sand accumulator 8. The wateroverflow discharged from cyclone separator 17 through its upper outletflows in line 6 to be combined with water from the gravity separator 11in line 18, to produce water flow passing through water line 14.

[0058] The staged cyclone separators 4″, 4′ are respectively providedwith flow control valves 30, 31 in lines 26, 28, respectively. The useof staged cyclone separators ensures that the gas and oil in overflowlines 26, 28, respectively are of high purity.

[0059] It will be noted that overflow line 26 from cyclone separator 4″passes directly to gas line 13, i.e. it “flies by” the gravity separator11. A cooling effect is produced by the expansion of gas in line 26,which produces condensate. Accordingly, a stripper 52 is provided inthis line, to remove the condensate from the gas stream.

[0060]FIG. 7 shows a particularly preferred embodiment effectively basedon the FIG. 5 embodiment but additionally employing gas/oil cycloneseparator 4″ such as in the FIG. 6 embodiment. The cut points forcyclone separators 4′, 4″, 17 and 22 are indicated at [4′], [4″], [17]and [22] in FIG. 7a. Since all remaining elements of the desanding planthave been described for the FIGS. 4 and 5 embodiments, no furtherdescription of the FIG. 7 plant is required. This embodiment combinesthe described advantages of the desanding plants according to FIGS. 5and 6. It is remarked that line 18 is provided purely as a contingencyagainst water unexpectedly collecting in gravity separator 11.

[0061] Referring now to FIG. 8, the desanding plant shown is similar tothat according to FIG. 7, but contains a modification. Specifically,line 18 is connected to line 21, so as to combine the water dischargedfrom gravity separator 11 with the oily water discharged as overflowfrom cyclone separator 17. Therefore, any oil in line 18 will beseparated front the water by oil/water separator 22.

[0062] In all of the described embodiments, the flow control valves inthe lines for overflow from the respective cyclone separators are set todetermine the separation (cut) points of the separators. Specifically,adjusting the setting of the valve changes the flow rate through thatvalve, and hence the input pressure to the cyclone, which determines theseparation point. Each flow control valve may be set manually. However,it is preferred that the getting is done automatically by a suitablecontrol system. FIG. 8 also indicates one possible manner in which thecontrol valves 20, 25, 30, 31 may be controlled automatically.

[0063] Specifically, connected in series with each control valve 20, 30,31 in the corresponding overhead line for overflow from the associatedcyclone separator is a multiphase flow measuring device 32, 33, 34,respectively. Each such flow measuring device provides output signalsrepresenting the flowrate of the stream concerned and the volumepercentages present of the constituent parts of that stream. Inaddition, a water analyser 35 is connected in water line 24/14 toprovide an output signal representing the oil content of the producedwater and multiphase flow measuring devices 36, 37, 38 are provided inoil, gas and water lines 12, 13, 14 to monitor the quality of therespective streams, by providing output signals representing the volumepercentage present of each constituent, and the flowrate of the streamsconcerned. Pressure sensors may also be provided, such as pressuresensors 41 to 49 in lines 3, 26, 27, 28, 16, 21, 50, 14 and 23,respectively. Optionally temperature, sensors may also be provided tomeasure the temperatures of selected ones of the streams.

[0064] A control unit 40, such as a computer, receives the outputsignals from the flow measuring devices 32-35 and 36, 37 and 38, thewater analyser 35, and the pressure and temperature sensors, whenpresent, and in dependence on the values of these output signals,adjusts the settings of the control valves 20, 25, 30, 31, so as tooptimise the quality of the controlled streams, and hence the quality(purity) of the oil, gas and water streams 12, 13, 14.

[0065] One preferred way of controlling the flow control valves is onthe basis of primary and secondary data sources as set out in thefollowing table. Cyclone Controlled Data Source separator by PrimarySecondary Objective 4 ″ 30 + 31 33 + 34 37 Gas quality 4 ′ 31 + 20 34 +32 36 Oil quality 17 31 + 20 34 += − Water quality 22 20 + 25 32 + 35 +38 − Water quality

[0066] Cyclone separator 4″ will be controlled to optimise the gasquality by adjusting valves 30 and 31. Flow measuring device 33 measuresthe gas flow and oil and water contents in the overhead stream fromseparator 4″. Flow measuring device 34 will monitor the gas contents inthe overhead stream (oil stream) from separator 4″. Secondary controlinput will come from flow measuring device 37 to set the initial targetgas flowrate.

[0067] Separator 4′ will be controlled to optimise the oil quality byadjusting valves 31, 20. Flow measuring device 34 measures the oil flowand gas and water contents in the overhead stream from separator 4′.Flow measuring device 32 will monitor the oil contents in the overheadstream (produced water stream) from separator 17. Secondary controlinput will come from flow measuring device 36 to set the initial targetoil flowrate.

[0068] Separator 17 will be controlled to optimise the produced waterquality by adjusting valves 31, 20. As already stated, flow measuringdevice 34 measures the oil flow and gas and water contents in theoverhead stream from separator 4′. Flow measuring device 32 will monitorthe oil contents in the overhead stream (produced water stream) fromcyclone separator 17.

[0069] Separator 22 will be controlled to optimise the produced waterquality by adjusting valves 20, 25. Flow measuring device 32 measuresthe water flow and oil contents in the overhead stream from separator17. Produced water analyser 35 will monitor the oil contents in theproduced water and flow measuring device 38 will measure the producedwater flowrate.

[0070] Measured data will be collected by the flow control computer 40.The optimum setting of the control valves will be evaluated based on theflowrate and quality data and, in addition, on the desired output, wherepreferences are operator dictated to suit downstream enhancementequipment.

[0071] Suitable algorithms for use in effecting the desired control arenumerous and will be known to those skilled in the art of controlsystems. Therefore, these will not be described in detail.

[0072] The constructionally simpler embodiments according to FIGS. 3 to6 may be controlled automatically, in a similar manner. By way ofexample, a suitable manner of control for various embodiments will nowbe described.

[0073] In the FIG. 3 embodiment, the cyclone separator 4′ is intended toproduce water (and sand) in the underflow that is of high quality, i.e.it contains substantially no oil or negligible oil. Therefore, theseparation point of this separator is set to produce a small amount ofwater in the overflow line 10, and the stream in this line is monitored,using a flow measuring device, for the presence of water. Providingwater is detected in this line, then it is known that there will be noor negligible oil in the line 16 since the separation point of thecyclone separator effects separation according to densities above andbelow the separation point. The small amount of water in the overflowfrom oil/water separator 4′ is removed in gravity separator 11.

[0074] A flow measuring device is also included in line 6 to monitor thesand properties present. The computer responds to the output signalsfrom the flow measuring devices in lines 10 to 16 to control theseparation points of the cyclone separator 4′, 17 such that the waterquality in line 16 is optimised (no oil) and the water in line 6contains no sand.

[0075] As for the FIG. 4 embodiment, the flow measuring devices in lines6, 10 are used to monitor the respective overflow streams, and inparticular the overflow produced by cyclone separator 4′ for the(substantial) absence of water and that from cyclone separator 17 forthe (substantial) absence of sand.

[0076] Turning now to the FIG. 5 embodiment, the cyclone separator 4′ isset to produce a high quality of gas and oil overhead stream, byaccepting a small amount of oil in the underflow stream. Cycloneseparator 17 is set to remove all the sand present, and cycloneseparator 22 then separates the stream consisting of the remainingconstituents into oil and water. The emphasis will be an the quality ofthe produced gas and water. A flow measuring device would be used inline 16 to detect oil content, a flow measuring device in line 21 woulddetect sand content and a flow measuring device in line 23 would detectwater content. The computer would respond to the output signals fromthese flow measuring devices to set the separation points of the cycloneseparators in the manner described above to optimise the separationprocess.

[0077] In FIG. 6, cyclone separator 4″ is set to produce a high qualitygas overhead stream. Therefore, it is necessary to ensure that theunderflow from cyclone separator 4″ contains, a minor proportion of gas.For this purpose, a flow measuring device is used in line 27 to checkfor the presence of gas. Cyclone separator 4′ is set to produce waterand sand only (substantially no oil present) as underflow, and a flowmeasuring device is used in overflow line 26 to check for the presenceof water. The cyclone separator 17 is set to remove all the sand,leaving high quality water in the overflow. A flow measuring device inline 6 checks for the presence of sand. The computer sets the separationpoints of the cyclone separators according to the measurement signals itreceives from the flow measuring devices.

[0078] The FIG. 7 embodiment is essentially the same as the FIG. 8embodiment and a corresponding manner of control may be used.

[0079] It is stressed that the above described ways of automaticallyoptimising the settings of the cyclone separators are merely exemplaryand other possible ways of controlling the separation points of thecyclone separators will be apparent to the skilled addressee.

[0080] The manner of automatic control described above in the severalembodiments involves controlling overhead flowrates, so as to optimisethe settings of the cyclone separators. However, the principal parametercontrolled could instead be the pressures of the corresponding stream,for example.

[0081] A preferred approach to the design of the control system,specifically in relation to each cyclone separator, is to set thequality of one of the two produced streams (overflow and underflow)according to the content in the other produced stream of the mainconstituent (s) forming the first stream. Thus, for example, in the FIG.6 embodiment, the quality of water in the underflow line 16 from cycloneseparator 4′ (quantified in terms of how little oil is present in thatline) is set in dependence on the measured amount of water present inthe oil stream leaving the separator 4′ as overflow in line 28. If wateris present in line 28, then there can be no oil present in line 16 sincewater has a higher density than oil. Such design philosophy may be usedin the control system used in all embodiments of the invention. However,such manner of control is disclosed purely by way of example, and itwill be appreciated that other types of control known to the skilleddesigner, alone or in combination, may be employed.

[0082] From the foregoing description, it will be appreciated that thedisclosed desanding plants offer many advantages. In particular, thepurity of the output streams (gas, oil and water) is improved ascompared with results achieved with the conventional plant according toFIG. 1, thereby enhancing the value of the oil and gas products andenabling the recovered water to be reused with minimal or no furtherpurification, depending on the required use for the water. For example,it could be used for reinjection into the well without any pre-treatmentto improve purity.

[0083] Furthermore; the erosive effect of the entrained sand isminimised since it is separated from the wellstream at or near the pointof entry into the desanding plant. Accordingly, the gravity separator isprotected from erosion and the supply line leading to it is alsoprotected. The cyclone separator(s) exposed to the entrained sand can befitted with erosion resisting replaceable liners.

[0084] By positioning the desanding plant on the sea bed adjacent theinlet manifold 1, the length of line 3 can be shortened, therebyminimising the effect of erosion of this line caused by the entrainedsand. In addition, since the demanding plant is located at the point ofhighest pressure, the gas phase is compressed to occupy the minimumspace also to go into solution in the stream, thereby reducing the sizeof the components of the plant for any given operating capacity.

[0085] Another important advantage of the desanding plants described isthat they achieve coalescence of the gas bubbles in the stream and alsoof the dispersed phase (generally oil droplets of an oil-in-wateremulsion but it would be water droplets when a water-in-oil phasemixture is present).

[0086] In all described embodiments, the oil, gas and water to bedischarged from the gravity separator may be removed continuously orperiodically. Furthermore, the oil and gas lines 12, 13 may form a pipebundle, optionally along with other lines to and/or from the well,directed in the form of a riser to a shore location or to an offshoreplatform or the like.

[0087] Whilst the preferred application is to separating theconstituents of a production wellstream with enhanced recovery due towater injection in the well, it is not essential that water injection beused. Where no gas phase is present, the gravity separator could bevented to atmosphere. Furthermore, in the case of the embodimentsaccording to FIGS. 6, 7 and 8, the first cyclone separator of thetwo-stage cyclone separators would not be needed. Where the hydrocarbonproduction fluid mixture in the stream comprises three or more phases(liquid phases, or a gas phase and liquid phases as the remainingphases), a complete, separation of the stream into its constituents maybe achieved by including one or more additional cyclone separatorstages, as appropriate.

[0088] It will be appreciated that in the embodiments described, eachcyclone separator achieves separation of the production fluid into twostreams of different densities, each stream containing one or moreconstituents. Furthermore, the separation point is set according tochoice to achieve the desired separation of the constituents, by settingthe input pressure at a desired value by adjustment of the setting ofthe control valve in the overflow line from the cyclone separator.

[0089] Finally, it is remarked that of course no cyclone separator canachieve one hundred percent exact separation as between constituents ofits input stream, irrespective of how the cut points of the individualcyclone separators are set. Inevitably, trace amounts of constituentsintended to exit primarily from one or the other outlet will be includedin the flow from the other outlet. Even though ouch trace constituentsare present, they are not mentioned or defined in this specification orits claims, except when specifically described.

1. A method of separating a hydrocarbon production fluid stream from aproduction well, which stream contains a multi-phase mixture comprisingoil and water and particles entrained therein, into individualconstituents of said production fluid, said mixture being rich in oil,said method comprising the steps of passing the stream into a firstcyclone separator (4′) having a cut point corresponding to a densitybetween the densities of the oil and heavier water to produce anunderflow that is rich in heavier water and particles and an overflowthat is rich in oil, passing the overflow from the first cycloneseparator (4′) to a first separation means (11) to effect a separationinto oil and water, and passing the underflow to a second separationmeans (17) to effect a separation of the underflow into a flow that isrich in water on the one hand and particles on the other hand.
 2. Amethod according to claim 1, wherein the stream is passed into the firstcyclone separator (4′) to produce an underflow that is rich in water andparticles on the one hand and an overflow that is rich in oil on theother hand.
 3. A method according to claim 1, for which the streamfurther includes a hydrocarbon gas phase, wherein the stream is passedinitially into a second cyclone separator (4″) to effect a separationinto an overflow that is rich in gas on the one hand and an underflowthat is rich in multi-phase mixture and entrained particles on the otherhand, the underflow rich in multi-phase mixture and particles then beingintroduced as feed into the first cyclone separator (4′).
 4. A methodaccording to any preceding claim, wherein a separation of the underflowfrom the first cyclone separator (4′) is effected by a third cycloneseparator (17) of the second separation means into an overflowcontaining a major proportion of water and substantially no particles,and an underflow containing a minor proportion of water and particles,and wherein a separation of the underflow from the third cycloneseparator is effected by a particle accumulator (8) of the secondseparation means into particles on the one hand and water on the otherhand.
 5. A method according to any preceding claim, wherein the firstseparation means (11) effects separation into oil, water and, whereapplicable, gas by gravity separation.
 6. A method according to claim 4,wherein the first cyclone separator (4′) has its cut point correspondingto a density closer to that of the water so that the overflow from thefirst cyclone separator (4′) contains a minor proportion of water andits underflow contains substantially no oil, and wherein the overflowfrom the third cyclone separator (17) is combined with the waterproduced by the first separation means (11).
 7. A method according toclaim 4, wherein the first cyclone separator (4′) has its cut pointcorresponding to a density closer to that of the oil so that theunderflow from the first cyclone separator (4′) contains a minorproportion of oil and its overflow contains substantially no water, andwherein the overflow from the third cyclone separator (17) is combinedwith overflow from the first cyclone separator (4′).
 8. A methodaccording to any one of claims 1 to 6, wherein the first cycloneseparator (4′) has its cut point corresponding to a density closer tothat of the oil so that the underflow from the first cyclone separatorcontains a minor proportion of oil and its overflow containssubstantially no water, a separation of the underflow from the firstcyclone separator is effected by the second separation means (17) into aflow of water containing said minor proportion of oil on the one handand particles on the other hand, and a separation of the flow from thesecond separation means (17) is effected by a fourth cyclone separator(22) essentially into water on the one hand and oil on the other hand.9. A method according to any preceding claim, wherein measurements aremade indicative of the quality of one output stream from a said cycloneseparator and the measurements are used to control a parameter of theoverflow from that cyclone separator to increase the quality of oneoutput stream or the other from that separator.
 10. A method accordingto claim 9, wherein said parameter is the flow rate of the overflow. 11.An installation for separating hydrocarbon production fluid comprising amulti-phase mixture of oil and water and particles entrained thereininto individual constituents of said production fluid, said hydrocarbonfluid being rich in oil, comprising a production well for producing saidhydrocarbon production fluid stream, a first cyclone separator (4′)connected to receive said production fluid stream as feed from saidproduction well, said cyclone separator having a cut point correspondingto a density between the densities of the oil and water to produce anunderflow that is rich in water and particles and an oil-rich overflow,a first separation means (11) arranged to receive the overflow from thefirst cyclone separator (4′) as feed and to separate the same intolighter density liquid and heavier density liquid and a secondseparation means (17) for effecting a separation of the underflow into awater-rich flow on the one hand and particles on the other hand.
 12. Aninstallation according to claim 11 and located on a seabed adjacent aproduction fluid wellstream manifold.